Heretofore, it has been required for a material of a diaphragm used in a subwoofer or the like that the material concerned has a small density and a high Young's modulus (rigidity), a moderate internal loss, and an environment resistance performance. Since a diaphragm made of polypropylene as an olefin system resin is excellent in an environment resistance, especially, durability, and is fine in an external appearance property, and is moderately large in an internal loss, and is also fine in a physicality balance in terms of a diaphragm for a speaker, the diaphragm made of polypropylene is frequently used next to a paper.
However, since in the diaphragm made of polypropylene, a specific gravity of polypropylene is 0.9 [g/cm3] which is larger than that of the paper, and has also a low Young's modulus, the rigidity is increased by reinforcement using a filler such as a carbon fiber. In this case, however, the specific gravity becomes larger. For this reason, the diaphragm made of polypropylene becomes heavier than the paper. Thus, in the diaphragm made of polypropylene, a sensitivity is reduced and an energy in a high-frequency band is also hard to output.
On the other hand, a diaphragm having a multi-layer structure is proposed which includes a first diaphragm and a second diaphragm which are made of materials, respectively, different from each other (for example, Patent Document 1).